The Mid-Infrared Extinction Law and Its Variation in the Coalsack Nebula

Abstract

In recent years the wavelength dependence of interstellar extinction from the ultraviolet (UV), optical, through the near-and mid-infrared (IR) has been studied extensively. Although it is well established that the UV/optical extinction law varies significantly among the different lines of sight, it is not clear how the IR extinction varies among various environments. In this work, using the color-excess method and taking red giants as the extinction tracer, we determine the interstellar extinction A λ in the four Spitzer/IRAC bands in [3.6], [4.5], [5.8], [8.0] µm (relative to A Ks , the extinction in the 2MASS K s band at 2.16 µm) of the Coalsack nebula, a nearby starless dark cloud, based on the data obtained from the 2MASS and Spitzer /GLIMPSE surveys. We select five individual regions across the nebula that span a wide variety of physical conditions, ranging from diffuse, translucent to dense environments, as traced by the visual extinction, the Spitzer/MIPS 24 µm emission, and CO emission. We find that A λ /A Ks , the mid-IR extinction relative to A Ks , decreases from diffuse to dense environments, which may be explained in terms of ineffective dust growth in dense regions. The mean extinction (relative to A Ks ) is calculated for the four IRAC bands as well, which exhibits a flat mid-IR extinction law, consistent with previous determinations for other regions. The extinction in the IRAC 4.5 µm band is anomalously high, much higher than that of the other three IRAC bands. It cannot be explained in terms of CO and CO 2 ices. The mid-IR extinction in the four IRAC bands have also been derived for four representative regions in the Coalsack Globule 2 which respectively exhibit strong ice absorption, moderate or weak ice absorption, and very weak or no ice absorption. The derived mid-IR extinction curves are all flat, with A λ /A Ks increasing with the decrease of the 3.1 µm H 2 O ice absorption optical depth τ ice .

Interstellar Extinction

“…The observations fit the WD2001 law well for R V = 5.5. The same IR extinction law, but with a peak at 4.5 µm, has been obtained by Wang, et al (2013), in the Coal Sack nebula and by , in the dense medium of the Large Magellanic cloud. These extinction peaks at 4.5 µm (W 2) are indicated in Fig.…”

“…The variations in the extinction law discussed here, especially in the IR, have been interpreted by Wang, et al (2013), in terms of three kinds of medium in the Galaxy: dense clouds of the disk and bulge with coarse dust and large IR extinction, the diffuse medium of the spiral arms with fine dust and low IR extinction, and the most rarefied medium between the arms and far from the galactic plane with an extinction law similar to that in dense clouds. Wang, et al (2013), emphasize the similarity of the first and third types of media: the temperature is very low, the gas is molecular, and there are few charged particles.…”

“…Wang, et al (2013), emphasize the similarity of the first and third types of media: the temperature is very low, the gas is molecular, and there are few charged particles. It is evident that in dense clouds, amalgamation of dust particles predominates over their breakup, in the spiral arms breakup predominates over amalgamation because of the high temperature, and in the most rarefied medium there are no processes leading to breakup of dust particles.…”

Interstellar Silicon Depletion and the Ultraviolet Extinction

“…2 to extrapolate the UV extinction at λ Weingartner & Draine (2001;WD01) for the R V = 3.1 diffuse ISM (see Figure 16 in Li & Draine 2001). This is justified since the near-and mid-IR extinction at λ > 0.9 µm does not seem to vary much among different environments (see Wang et al 2013. In Table 1 we tabulate the wavelength-integrated extinction for each sightline.…”

“…To examine this, we explore how the mean grain sizes derived in §5 from fitting the observed extinction curves vary with R −1 V . 7 The extinction law at λ 0.90 µm appears to vary very little with environments (e.g., see Martin & Whittet 1990, Wang et al 2013). …”